The present invention relates to anaesthetic vaporisers which are devices for mixing the vapour of a volatile liquid anaesthetic agent with a carrier gas (which term is to be understood to include gas mixtures), for subsequent administration to a patient.
Throughout this specification the term "anaesthetic" is intended to embrace both anaesthetics and analgesics.
Anaesthetic vaporisers are known in which the carrier gas supplied to the vaporiser is divided into two streams. One stream is directed through a vaporising chamber containing volatile liquid anaesthetic where it becomes saturated with the vapour of the anaesthetic. The second stream by-passes the vaporising chamber. The two streams subsequently reunite downstream of the vaporising chamber and then pass through an outlet of the vaporiser for administration to the patient.
Such anaesthetic vaporisers are know generally as "by-pass" vaporisers and an example of such a by-pass vaporiser is described in UK Pat. No. 968054.
UK Pat. No. 968054 describes a by-pass vaporiser in which the concentration of anaesthetic vapour in the gas leaving the vaporiser is controlled by a valve arrangement which is capable of varying the resistance to gas flow of the second or by-pass stream. The valve arrangement has the effect of altering the proportions in which the carrier gas is divided into said two streams. This known by-pass vaporiser has been found to be successful in medical applications in that the concentration of anaesthetic vapour in the gas leaving the vaporiser can be controlled accurately.
However, this known by-pass vaporiser does have the disadvantage that the valve arrangement is complicated and requires very accurate machining and fitting and is thus expensive to produce. Further, the vaporiser needs to be monitored by a skilled anaesthetist since the concentration of anaesthetic is infinitely variable between set limits.